I haven't heard of timber ridge but I am familiar with the Liberty brand and they seem right on par with a guality safe for their price.
The S&G lock is extremely common and is of high quality. The combo can be changed easily once the back of the lock is exposed. This change should be done by a locksmith if you desire to change it. It can be set to open on a one number combo if you desire.
The keyhole is to lock the dial in place to prevent tampering. It can be forced to turn however with enough force but the combo would still need to be run to get it open. S&G locks have an internal relocker built in. The Fact that it is an S&G tells me that you could retro fit a digital combo lock in its place later if you wanted one.
GRAVITY. It's not just a good idea. It's the LAW!